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<h1 ALIGN="CENTER">Issues Regarding DNS and Apache</h1>

<p>This page could be summarized with the statement: <i>don't require
Apache to use DNS for any parsing of the configuration files</i>.
If Apache has to use DNS to parse the configuration files then your
server may be subject to reliability problems (it might not boot), or
denial and theft of service attacks (including users able to steal hits
from other users).

<h3>A Simple Example</h3>

Consider this configuration snippet:

<blockquote><pre>
    &lt;VirtualHost www.abc.dom&gt;
    ServerAdmin webgirl@abc.dom
    DocumentRoot /www/abc
    &lt;/VirtualHost&gt;
</pre></blockquote>

<p>In order for Apache to function properly it absolutely needs
to have two pieces of information about each virtual host: the
<a href="mod/core.html#servername"><code>ServerName</code></a>
and at least one IP address that the server
responds to.  This example does not include the IP address, so Apache
must use DNS to find the address of <code>www.abc.dom</code>.  If for
some reason DNS is not available at the time your server is parsing its
config file, then this virtual host <b>will not be configured</b>.  It
won't be able to respond to any hits to this virtual host (prior to
Apache version 1.2 the server would not even boot).

<p>Suppose that <code>www.abc.dom</code> has address 10.0.0.1.  Then
consider this configuration snippet:

<blockquote><pre>
    &lt;VirtualHost 10.0.0.1&gt;
    ServerAdmin webgirl@abc.dom
    DocumentRoot /www/abc
    &lt;/VirtualHost&gt;
</pre></blockquote>

<p>Now Apache needs to use reverse DNS to find the <code>ServerName</code>
for this virtualhost.  If that reverse lookup fails then it will partially
disable the virtualhost (prior to Apache version 1.2 the server would not
even boot).  If the virtual host is name-based then it will effectively
be totally disabled, but if it is IP-based then it will mostly work.
However if Apache should ever have to generate a full URL for the server
which includes the server name then it will fail to generate a valid URL.

<p>Here is a snippet that avoids both of these problems.

<blockquote><pre>
    &lt;VirtualHost 10.0.0.1&gt;
    ServerName www.abc.dom
    ServerAdmin webgirl@abc.dom
    DocumentRoot /www/abc
    &lt;/VirtualHost&gt;
</pre></blockquote>

<h3>Denial of Service</h3>

<p>There are (at least) two forms that denial of service can come in.
If you are running a version of Apache prior to version 1.2 then your
server will not even boot if one of the two DNS lookups mentioned above
fails for any of your virtual hosts.  In some cases this DNS lookup may
not even be under your control.  For example, if <code>abc.dom</code>
is one of your customers and they control their own DNS then they
can force your (pre-1.2) server to fail while booting simply by deleting the
<code>www.abc.dom</code> record.

<p>Another form is far more insidious.  Consider this configuration
snippet:

<blockquote><pre>
    &lt;VirtualHost www.abc.dom&gt;
    ServerAdmin webgirl@abc.dom
    DocumentRoot /www/abc
    &lt;/VirtualHost&gt;
</pre></blockquote>

<blockquote><pre>
    &lt;VirtualHost www.def.dom&gt;
    ServerAdmin webguy@def.dom
    DocumentRoot /www/def
    &lt;/VirtualHost&gt;
</pre></blockquote>

<p>Suppose that you've assigned 10.0.0.1 to <code>www.abc.dom</code> and
10.0.0.2 to <code>www.def.dom</code>.  Furthermore, suppose that
<code>def.com</code> has control of their own DNS.  With this config
you have put <code>def.com</code> into a position where they can steal
all traffic destined to <code>abc.com</code>.  To do so, all they have to
do is set <code>www.def.dom</code> to 10.0.0.1.
Since they control their own DNS you can't stop them from pointing the
<code>www.def.com</code> record wherever they wish.

<p>Requests coming in to 10.0.0.1 (including all those where users typed
in URLs of the form <code>http://www.abc.dom/whatever</code>) will all be
served by the <code>def.com</code> virtual host.  To better understand why
this happens requires a more in-depth discussion of how Apache matches
up incoming requests with the virtual host that will serve it.  A rough
document describing this <a href="vhosts-in-depth.html"> is available</a>.

<h3>The "main server" Address</h3>

<p>The addition of <a href="host.html">non-IP-based virtual host
support</a> in Apache 1.1 requires Apache to know the IP address(es) of
the host that httpd is running on.  To get this address it uses either
the global <code>ServerName</code> (if present) or calls the C function
<code>gethostname</code> (which should return the same as typing
"hostname" at the command prompt).  Then it performs a DNS lookup on
this address.  At present there is no way to avoid this lookup.

<p>If you fear that this lookup might fail because your DNS server is down
then you can insert the hostname in <code>/etc/hosts</code> (where you
probably already have it so that the machine can boot properly).  Then
ensure that your machine is configured to use <code>/etc/hosts</code>
in the event that DNS fails.  Depending on what OS you are using this
might be accomplished by editing <code>/etc/resolv.conf</code>, or maybe
<code>/etc/nsswitch.conf</code>.

<p>If your server doesn't have to perform DNS for any other reason
then you might be able to get away with running Apache with the
<code>HOSTRESORDER</code> environment variable set to "local".  This all
depends on what OS and resolver libraries you are using.  It also affects
CGIs unless you use <a href="mod/mod_env.html"><code>mod_env</code></a>
to control the environment.  It's best to consult the man pages or FAQs
for your OS.

<h3>The _default_ Address</h3>

<p>Any address that happens to go to your webserver which doesn't match
the IP address of any of the webservers will be served from the "main" or
"default" server configurations.  The "main" server configuration consists
of all those definitions appearing outside of any VirtualHost section.
You may want instead to define a <code>&lt;VirtualHost _default_:*&gt;</code>
which returns 403 or 404 for all hits.  (The trailing <code>:*</code>
makes it apply to all ports, which is just a safety measure should you
begin using multiple <code><a href="mod/core.html#listen">Listen</a></code>
directives.)

<h3><a name="tips">Tips to Avoid these problems</a></h3>

<ul>
<li> use IP addresses in <code>&lt;VirtualHost&gt</code>
<li> use IP addresses in <code>Listen</code>
<li> use IP addresses in <code>BindAddress</code>
<li> ensure all virtual hosts have an explicit <code>ServerName</code>
<li> create a <code>&lt;VirtualHost _default_:*&gt;</code> server that
    has no pages to serve
</ul>

<h3>Appendix: Future Directions</h3>

<p>The situation regarding DNS is highly undesirable.  For Apache
1.2 we've attempted to make the server at least continue booting
in the event of failed DNS, but it might not be the best we
can do.  In any event requiring the use of explicit IP addresses in
configuration files is highly undesirable in today's Internet where <a
href="http://www.ietf.org/html.charters/pier-charter.html">renumbering
</a> is a necessity.

<p>A possible work around to the theft of service attack described above
would be to perform a reverse DNS lookup on the ip address returned by
the forward lookup and compare the two names.  In the event of a mismatch
the virtualhost would be disabled.  This would require reverse DNS to be
configured properly (which is something that most admins are familiar with
because of the common use of "double-reverse" DNS lookups by FTP servers
and TCP wrappers).

<p>In any event it doesn't seem possible to reliably boot a virtual-hosted
web server when DNS has failed unless IP addresses are used.  Partial
solutions such as disabling portions of the configuration might be worse
than not booting at all depending on what the webserver is supposed
to accomplish.

<p>As HTTP/1.1 is deployed and browsers and proxies start issuing the 
<code>Host</code> header it will become possible to avoid the use of
IP-based virtual hosts entirely.  In this event a webserver has no requirement
to do DNS lookups during configuration.  But as of March 1997 these
features have not been deployed widely enough to be put into use on
critical webservers.

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